A method of determining a rotational state of a three-phase alternating voltage supply which is connected to a converter and rotating in an uncontrolled manner, wherein the converter is connected to an intermediate voltage circuit and comprises phase-specific upper and lower controllable switches, which are connected in series between the intermediate voltage circuit, free-wheeling diodes connected in parallel with each of the controllable switches, and resistive circuits connected in parallel with the lower controllable switches. The method comprises the steps of selecting a voltage limit, comparing the voltages of the alternating voltage supply in the resistive circuits with the voltage limit, determining a first time instant when any of the voltages of the resistive circuits crosses the voltage limit, determining a second time instant when any of the voltages of the resistive circuits subsequently crosses the voltage limit, determining the phase sequence of the converter output phases from the phases that crossed the voltage limit at the first and second time instants, determining the frequency of the alternating voltage source from the determined time instants, the method being applicable when the voltage of the intermediate circuit is equal to or higher than the voltage of the alternating voltage source.